In the context of the present application the term “pairwise symmetrical, optically effective surfaces” encompasses all such surfaces, i.e. not only the progressive power spectacle lenses already mentioned as an example. Instead, the invention may be used for all applications on optically effective surfaces being used in pairs, for example for lenses and mirrors as are used in binocular applications, e.g. telescopes, microscopes, and the like.
U.S. Pat. No. 6,991,525 describes a method and a tool head of the type specified at the outset. This prior art method uses a tool head having an essentially tapered main body. The main body axis coincides with the tool head axis of rotation. The larger terminal plane of the main body lies at the free end of the tool head. Two equal lathing tools being offset relative to one another by 180° are located at the periphery of the larger terminal plane and are equally oriented in a circumferential direction of the periphery. Further, there are eight milling tools distributed along the periphery. The cutting edges of all these tools are rigidly bolted at their respective positions. No means for adjusting the position of the cutting edges are provided.
The spectacle lens to be machined is positioned in a holder being chucked in a work piece spindle. The work piece spindle rotates the spectacle lens about a first axis and is further adapted to be displaced along that first axis. The tool head is chucked in a tool spindle having a second axis which is inclined relative to the first axis of the work piece spindle. The tool is adapted to be rotated about the second axis at a high rotary speed. The tool spindle, further, is adapted to be displaced along a third axis extending perpendicular to the first axis.
For machining the spectacle lens one proceeds in two steps. Firstly, in a first operating step the substantially major portion of the overmeasure of a spectacle lens blank is removed by milling. Secondly, in a second operating step the fine machining and the shaping of the surface is effected so as to generate the desired optical plane, for example a prescription plane. Subsequently, the spectacle lens may be polished or otherwise coated, as known per se.
For effecting the first operational step, the tool head is brought into rotation about the second axis. By displacing the spectacle lens along the first axis and by displacing the work piece spindle along the third axis, a point of engagement of the milling tool at the periphery of the surface to be machined is set. The spectacle lens is rotated about the first axis in a predetermined direction of rotation, and the tool head is displaced continuously along the third axis, such that the point of engagement moves along a narrow spiralled path on the surface towards a central point. The material is, thereby, removed by milling. By superimposing a movement of the spectacle lens along the first axis, a coarse convex or concave shaping may be effected simultaneously, which, however, is not the exact shaping of the desired free-form surface.
For the second operational step the tool head is rotated only once about a finite angle until a point of engagement of a lathing tool is at a desired peripheral position, whereafter it is rotated no more. The spectacle lens is now again rotated about the first axis in a predetermined direction of rotation, and the tool head is continuously displaced along the third axis such that the point of engagement again moves along a narrow spiralled path on the surface towards the center point thereof. The material is now removed by lathing. In this second operational step an elevation function along the spiralled path is set by superimposing a movement of the spectacle lens along the first axis. The elevation function corresponds to the desired free-form shape of the optical surface.
Now, during the manufacture of such spectacle lenses, the free-form surfaces for the right side and for the left side lens of equal optical powers are essentially mirror-symmetrical. In conventional manufacturing processes, the machining of all spectacle lenses, i.e. the right side lenses as well as the left hand lenses, is effected within the same apparatus as described before, and is effected in the same manner, i.e. always along the same direction of rotation of the spectacle lens about the first axis.
During the chip-removingly machining of spectacle lenses one is always confronted with inherent form defects because the machining system, consisting of spindles, mounts, etc., is elastic, and, therefore, due to inertial forces exerted by moved elements, positioning errors occur during changes in position. This holds true in particular for the setting of the elevation function. The spectacle lenses, namely, are rotated in practice at rotational speeds within such high rotational speed ranges (typically several 100 rpm) about the first axis that non-negligible form defects occur due to insufficient dynamic behaviour when the spectacle lens is simultaneously displaced along the first axis.
The machining system is non-symmetrical in the meaning that like form defects would occur with inverted sign when the same elevation function is run through in the opposite direction. During the conventional machining of spectacle lenses, as described above, this results in practice in unequal form defects for the right side and the left side spectacle lenses. This must be prevented by substantial correctional measures, i.e. by shaping lead values that are complicated to compute.
The prior art tool, further, has the disadvantage that due to the rigid arrangement of the tools on the tool head, one cannot produce defined conditions of engagement.